Due to the recent reinforcement of environmental regulations, it is essentially required that the content of environmental contaminants emitted from engines is reduced, and in order to solve the problem, it is necessary to raise the combustion temperature by increasing the explosion pressure of the engine. When the explosion pressure of the engine is increased as described above, the strength of the engine cylinder block and head constituting the engine needs to be high in order to withstand the explosion pressure.
A material currently used for the engine cylinder block and head is flake graphite cast iron to which a trace of alloy iron such as chromium (Cr), copper (Cu) and tin (Sn) is added. The flake graphite cast iron has excellent thermal conductivity and vibration damping capacity and a trace of added alloy iron, and thus has excellent castability as well as low chilling probability. However, since the tensile strength is in a range from approximately 150 to 250 MPa, there is limitation in using the flake graphite cast iron for an engine cylinder block and head which requires explosion pressure of more than 180 bar.
Meanwhile, a material for the engine cylinder block and head for withstanding an explosion pressure more than 180 bar is required to have high strength such as a tensile strength of approximately 300 MPa. For this purpose, a pearlite stabilizing element such as copper (Cu) and tin (Sn) or a carbide production promoting element such as chromium (Cr) and molybdenum (Mo) needs to be further added, but since the addition of such an alloy iron potentially induces the chilling tendency, there is a problem of increasing the likelihood that chills occur at a part such as a thin-walled part of an engine cylinder block and head having a complicated shape. When a large number of chills occur, brittleness of a material increases and the material becomes vulnerable to impact, and there are problems in that physical properties deteriorate and the workability deteriorates.
Recently, compacted graphite iron (CGI) cast iron having excellent castability, vibration damping capacity and thermal conductivity of flake graphite cast iron and simultaneously satisfying a high tensile strength of 300 MPa or more has been applied as a material for an engine cylinder block and head having a high explosion pressure. In order to make a CGI cast iron having a tensile strength of 300 MPa or more, high-quality pig iron having a low content of impurities such as sulfur (S) and phosphorus (P), and a molten material need to be used, and it is necessary to precisely control magnesium (Mg) which is a graphite-spheroidizing element. However, since it is difficult to control magnesium (Mg) and magnesium is very sensitive to a change in melting and casting conditions, such as pouring temperature and pouring rate, it is highly likely that material defects and casting defects of CGI cast iron occur, and there is a problem in that the costs of production increase.
Since CGI cast iron has relatively worse workability than flake graphite cast iron, when an engine cylinder block and head is prepared using CGI cast iron, processing is not performed in a processing line dedicated to the existing flake graphite cast iron and it is necessary to change the processing line into a processing line dedicated to CGI cast iron. Therefore, there is a problem concerning the occurrence of enormous facility investment costs.